1. Field of the Invention
This invention relates in general to charge-coupled devices and to memories and in particular to a novel charge-coupled memory which has fast read-in and read-out time and which is capable of storing large quantities of information.
2. Description of the Prior Art
Charge-coupled semiconductor devices are described in the Bell System Technical Journal of April 1970 at pages 587-600 by W. S. Boyle and G. E. Smith. The article appearing in Electronics of May 11, 1970 at pages 112-118 also describes such prior art devices. These devices primarily store minority carriers in a spatially defined depletion region and shift such stored charges linearly through the devices by phased input signals. Such devices comprise an insulating layer formed over a substrate of an N- or a P-type conductive monocrystalline material. The insulating material might, for example, comprise silicon-dioxide. A number of electrodes are arranged in a row and are supplied with different cyclically varying alternating voltage. The voltages are selected relative to the barrier-free electrode of the semiconductor member such that space charge zone in the form of so-called depletion layers will be produced in the semiconductor at the boundary with the insulating layer.
For a simple two-electrode arrangement, depletion zones will be produced directly adjacent to the insulating layer beneath the electrodes. The fixed space charges will be positive if the semiconductor substrate is N-type semiconductor material and will be negative if the substrate is P-type semiconductor material. The space charge zone thus depends on the concentration of doping in the semiconductor and on the magnitude of the applied voltages to the electrodes. Minority charge carriers produced in the semiconductor substrate will have the same sign as the space charge and will collect at the boundary surface beneath the insulating layer. It is possible to shift this movable charge consisting of minority charges from the area below one of the electrodes to under the area below the other electrode, or, vice-versa, by means of varying the potentials which are applied to the two electrodes. Such principle is used to switch charges in the prior art devices.
The prior art arrangements comprise a number of insulating electrodes arranged in a line or chain to form a two-dimensional diode raster. It is possible, due to corresponding potentials applied to the individual electrodes which periodically vary with respect to time, to shift a charge fed into the first electrode which can consist of minority charge carriers successively down the electrode chain to an output electrode at the other end of the chain. Such charge is gradually moved from electrode to electrode and finally appears as an output signal at the output electrode which feeds it to an output circuit for utilization and evaluation.
Thus, with charge-coupled devices of prior art, a relatively long transport chain must be traversed by the intelligence before it can be read out. This has the following drawbacks: First, since a certain percentage of the information is lost during each transport step from electrode to electrode, the length of the chain is limited due to the required signal-to-noise ratio of the output signal; second, with such devices it is not possible to individually read an individual element without changing the information content of the succeeding elements (It would be very advantageous to simultaneously read the output of all elements in such devices); third, the speed of reading a linear matrix of the prior art is very slow due to the required serial reading.